Overview of Google crawlers (user agents)

"Crawler" is a generic term for any program (such as a robot or spider) that is used to automatically discover and scan websites by following links from one webpage to another. Google's main crawler is called Googlebot. This table lists information about the common Google crawlers you may see in your referrer logs, and how they should be specified in robots.txt, the robots meta tags, and the X-Robots-Tag HTTP directives.

The following table shows the crawlers used by various products and services at Google:

• •User agent token is used in the User-agent: line in robots.txt to match a crawler type when writing crawl rules for your site. Some crawlers have more than one token, as shown in the table; you need to match only one crawler token for a rule to apply. This list is not complete, but covers most of the crawlers you might see on your website.
• •Full user agent string is a full description of the crawler, and appears in the request and your web logs.

User agents in robots.txt

Where several user-agents are recognized in the robots.txt file, Google will follow the most specific. If you want all of Google to be able to crawl your pages, you don't need a robots.txt file at all. If you want to block or allow all of Google's crawlers from accessing some of your content, you can do this by specifying Googlebot as the user-agent. For example, if you want all your pages to appear in Google search, and if you want AdSense ads to appear on your pages, you don't need a robots.txt file. Similarly, if you want to block some pages from Google altogether, blocking the user-agent Googlebot will also block all Google's other user-agents.

But if you want more fine-grained control, you can get more specific. For example, you might want all your pages to appear in Google Search, but you don't want images in your personal directory to be crawled. In this case, use robots.txt to disallow the user-agent Googlebot-image from crawling the files in your /personal directory (while allowing Googlebot to crawl all files), like this:

User-agent: Googlebot
Disallow:

User-agent: Googlebot-Image
Disallow: /personal

User-agent: Googlebot
Disallow: /

User-agent: Mediapartners-Google
Disallow:

User agents in robots meta tags

Some pages use multiple robots meta tags to specify directives for different crawlers, like this:

<meta name="robots" content="nofollow"><meta name="googlebot" content="noindex">

In this case, Google will use the sum of the negative directives, and Googlebot will follow both the noindex and nofollow directives. More detailed information about controlling how Google crawls and indexes your site.